This invention relates in general to gas turbine engines and, more particularly, to shims for use on gas turbine engines to prevent wear of selected engine surfaces.
As seen in FIG. 1, gas turbine engines typically include a compressor 10, a combustor 15 and a turbine 20 all contained within an engine case 25. Turbine 20 includes a plurality of stator vanes or nozzles 30, which are fixed with respect to engine casing 25, and a plurality of rotor blades 35 which are mounted on a rotary shaft 40. Conventional engines include an internal shroud within the interior of the engine to minimize the amount of air which bypasses the rotor blades. In this manner, as tight a fit as possible is provided between the top of the rotor blades and the engine case such that air does not undesirably go over the tip of the rotor blades and bypass the rotor blades.
Unfortunately, in conventional gas turbine engines, a large amount of engine case wear is observed at locations in the case where the shroud contacts the case. This occurs due to the high temperatures which are present in the engine. More particularly, when the engine is subjected to typical high temperature operating conditions of 1000-2500 degrees Fahrenheit, differential thermal expansion and corresponding relative movement is observed between the case and the shroud. In other words, the engine casing expands at a different rate than the shroud and those engine case locations where the shroud contacts the case are exposed to relative motion between these two parts. Undesired engine case wear thus occurs.
Another source of engine case wear is the differential thermal expansion of stator vanes or nozzles with respect to the case at case locations where such vanes are attached to the engine case.
Yet another source of engine casing wear is the relative motion between the case and shroud, and the case and stator vanes, which occurs due to high velocity air passing over the shroud and vanes.
Since the engine case is a very expensive component to replace, undesired engine case wear at case locations where the shrouds and vanes contact the engine case can cause significant maintenance expense.
It is known to dispose wear shims between 2 surfaces which are wearing with respect to each other, However, in gas turbine engines wear shims have thus far been attached to the engine case by an active retention members such as weldments and retaining pins. Such attachment techniques can make shim removal difficult and/or expensive.